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Author Topic: Trying to understand Quantum Entanglement  (Read 8190 times)

Offline Pikaia

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« on: 03/11/2010 12:44:37 »
I have been trying to understand why Quantum Entanglement is so spooky, I do not see the problem.

 If we pass a photon through a polariser then it will emerge with a certain polarisation, which is defined by an angle. The photon 'knows' what its polarisation is, because if it meets a second polariser at 90 degrees to the first it is certain to be absorbed, so the state is well-defined. However, someone who doesn't know that the photon has been through a polariser cannot determine the polarisation, all he can do is pass it through a filter and see what happens, but this does not tell him exactly what the polarisation was.

So a photon can have a well defined polarisation, but we cannot determine exactly what it is.

Now why can't entangled photons also have this same well defined but indeterminate polarisations when they are produced? Doesn't this remove the problem of superluminal communication? Why is this explanation incompatible with quantum theory?


 

Offline graham.d

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« Reply #1 on: 03/11/2010 13:25:14 »
I think you have the right idea. Quantum Entanglement does not allow superluminal communication. It does involve "Spooky" action at a distance but does not actually convey information faster than light. Roger Penrose explains this quite well in one of his popular and readable books.

I vaguely remember a quotation from (I think) Richard Feynman along the lines that "Anyone who claims to understand Quantum Mechanics doesn't understand Quantum Mechanics".
 

Offline JP

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« Reply #2 on: 03/11/2010 13:36:31 »
There's one other catch: before the photon hits the first polarizer, it's in a completely indeterminate polarization state.  It might be polarized to pass through without loss, or it might be polarized to be completely blocked.  Part of entanglement is that before you measure the photons, they can be written as having a probability of existing in two or more polarization states at once.
 

Offline Pikaia

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« Reply #3 on: 03/11/2010 17:09:10 »
  Part of entanglement is that before you measure the photons, they can be written as having a probability of existing in two or more polarization states at once.
If I send you a photon to examine, you do not know what its polarisation is so it makes reasonable sense for you to talk about different states and different probabilities for each.

However, since I passed the photon through a polariser I know exactly what its state is, so I do not need to do the same. As far as I am concerned it does not exist in two states at once, it has one state which I have specified exactly. Talking about the photon being in two states at once is therefore merely a statement about your own ignorance, which does not apply to me or to the photon.
 

Offline Soul Surfer

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« Reply #4 on: 03/11/2010 23:34:21 »
You appear to be missing the point.  Entanglement involves TWO particles with related properties not just the properties of a single particle being indeterminate until measured.  The really peculiar bit is that the second particle is also indeterminate until you measure the first one.  After that it is instantly determinate even if they are a long way apart. It effectively suggests that although particles may be a long way apart in the real dimensions that we can observe they are probably still close together in dimensions that we cannot observe.
 

Offline Pikaia

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« Reply #5 on: 03/11/2010 23:59:17 »
You appear to be missing the point.  Entanglement involves TWO particles with related properties not just the properties of a single particle being indeterminate until measured. 
The point I am trying to make is that I can create a photon with a well defined state by passing it through a polariser, so if I create a pair of entangled photons why can't they have equally well defined states? If a polariser gives photons a specific polarisation angle, then why doesn't a parametric down converter?

Also, suppose I simultaneously sent you two photons polarised orthogonally by using two polarisers. These are produced independently so they are not entangled, but how can you tell that they are not entangled? How would their behaviour differ from an entangled pair?
 

Offline jartza

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« Reply #6 on: 04/11/2010 19:43:46 »
Also, suppose I simultaneously sent you two photons polarised orthogonally by using two polarisers. These are produced independently so they are not entangled, but how can you tell that they are not entangled? How would their behaviour differ from an entangled pair?

These photon's polarization measurement results are less diverse than those other photon's, that are polarized both this way and that way.

 
 

Offline CPT ArkAngel

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« Reply #7 on: 05/11/2010 03:00:59 »
What if... During and or after the creation of Matter and Antimatter (BigBang creation), Matter particles appeared in our dimension and their Quantum entangled Antimatter particles went in a parallel Universe and dimensions... When you look at yourself in a mirror, you could see your twin of Antimatter in its parallel World... You see him invert from right to left with a vertical axis of antisymmetry... Every event you watch is simultaneous with your twin but antisymmetric...

I am just kidding but i am sure you will never look at yourself in a mirror the same way... ;D

http://en.wikipedia.org/wiki/Copenhagen_interpretation

see 4. EPR (Einstein–Podolsky–Rosen) paradox

« Last Edit: 05/11/2010 03:21:46 by CPT ArkAngel »
 

Offline JP

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« Reply #8 on: 05/11/2010 05:18:38 »
You appear to be missing the point.  Entanglement involves TWO particles with related properties not just the properties of a single particle being indeterminate until measured. 
The point I am trying to make is that I can create a photon with a well defined state by passing it through a polariser, so if I create a pair of entangled photons why can't they have equally well defined states? If a polariser gives photons a specific polarisation angle, then why doesn't a parametric down converter?
The difference is that passing photons through polarizers produces two separate photons that can be described independently.  Entangled photons cannot be described independently.  The polarization of one photon is connected to the polarization of the other photon. 


Quote
Also, suppose I simultaneously sent you two photons polarised orthogonally by using two polarisers. These are produced independently so they are not entangled, but how can you tell that they are not entangled? How would their behaviour differ from an entangled pair?
To see the difference, I'll give you an example.  Suppose you have 2 polarizers that put both photons in polarization state A half the time and in polarization state B the other half of the time.  Then if you want to describe the state you're getting at the other end, you have either two photons entirely in A or two photons entirely in B.  Either way, you only have one polarization state.

With entangled photons, a single pair of photons can be 50% in  state A and 50% in state B.  That single state can't be generated with just polarizers.

As for the physical difference between them, when you measure a bunch of these states, there are statistics from the entangled state--correlations between the two photons--which you can't get with pairs of polarizers.
 

Offline Pikaia

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« Reply #9 on: 05/11/2010 11:29:48 »
As for the physical difference between them, when you measure a bunch of these states, there are statistics from the entangled state--correlations between the two photons--which you can't get with pairs of polarizers.
In what way are the statistics different? For unentangled photons with orthogonal polarisation (chosen at random), passing through orthogonal polarisers, each photon has a 50% chance of being transmitted, so both will be transmitted 25% of the time, both are absorbed 25% of the time, and 50% of the time one is absorbed and one transmitted.

What are the percentages for entangled photons?
 

Offline JP

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« Reply #10 on: 05/11/2010 16:28:35 »
Ok.  Let's say you change the experimental setup slightly so that you create photons that always have the exact same polarization, and that you measure them with polarizers which are both oriented in the same direction.  Let's also think only of a single pair of photons at a time for now.

Let's say you create the photons first by using two independent polarizers.  If your measurement polarizers are aligned with the photon polarization, you see both photons 100% of the time.  If you rotate your measurement polarizers, sometimes only 1 photon through the polarizers, since one can be absorbed and the other passed.

The difference with entangled photons is that you can entangle them so that they are always either passed or blocked.  You will never see an event where only 1 photon is passed through the polarizers.

If you set up your experiment so that your polarizers are oriented at 45 degrees to your polarization direction, you'll see 25% two-hit detections, 50% 1-hit detection and 25% 0-hit detections for the unentangled photons, while you'll see 50% 2-hit detections and 50% no-hit detections for the entangled photon case. 

You can set up the experiment you described to do the same thing.  If you entangle your photons correctly, I believe you can make it so that you won't see any cases of only 1 photon arriving.
 

Offline Pikaia

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« Reply #11 on: 05/11/2010 16:43:23 »
If you set up your experiment so that your polarizers are oriented at 45 degrees to your polarization direction, you'll see 25% two-hit detections, 50% 1-hit detection and 25% 0-hit detections for the unentangled photons, while you'll see 50% 2-hit detections and 50% no-hit detections for the entangled photon case. 

I suspected that that is what happens, but I have not been able to find anything on the internet which spells it out in these simple terms. All the explanations I have found are confusing and over-complicated so I was unclear about whether that was what they were trying to say (and I suspect that some of them are wrong anyway).

Thank you for clarifying it all! Yes, it is spooky!
 

Offline CPT ArkAngel

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Offline sciconoclast

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« Reply #13 on: 10/11/2010 18:06:47 »
As weird as it is this is how I see it.

There are many interpretations of quantum theory but the Copenhagen interpretation is sited as the predominate interpretation when ever physicist are polled.   In this interpretation an event that eventually leads to two mirror image photons does not initially generate two photons.   There is instead an expanding quantum field that contains multiple, changing, probabilities for the two photon symmetries. 

When the probabilities defining a left handed photon are highest in one direction within the quantum field they are also highest for a right handed photon in the opposite direction.   Entanglement is intrinsic to the quantum field and when one photon is tested for at its highest probability for occurrence at that point both that photon and its mirror photon come into existence as photons instantly and simultaneously.  This collapses the quantum field and ends entanglement.

Einstein claimed that the E.P.R. experiments disproved the concept of quantum field collapse because in his view nothing could occur instantaneously across time and distance.   Bohr on the other hand saw the experiments as proof that things could.

Quantum theory, unlike quantum mechanics, is still only theory.   My personal bias is that pikaia's statement that photons with properties are generated in the initial event is, although contrary to mainstream physics, correct.
I do not dislike quantum theory because of its seeming absurdity ( after all the idea that the earth wasn't flat was considered an absurdity at one time ) but because it does not meet experimental results.

I have posted several of my own experiments on this site that contradict the quantum field concept and there have been experiments by others with similar results showing up in recent papers.

        On the other hand I could of course be completely wrong.   
 

Offline JP

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« Reply #14 on: 11/11/2010 03:17:33 »
Quantum theory, unlike quantum mechanics, is still only theory.   My personal bias is that pikaia's statement that photons with properties are generated in the initial event is, although contrary to mainstream physics, correct.

Everything in science is "only a theory."  Some theories are more correct than others, and entanglement is pretty solidly established.  Statistics are generated here that cannot be generated with classical mechanics.  QM might not be the full story in the end, but so far it seems to explain all the experiments while classical theory cannot.
 

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Offline Geezer

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« Reply #15 on: 11/11/2010 05:01:54 »
Shrunk
Everything in science is "only a theory." 

I never cease to be amazed at how often the "it's only a theory" argument is used. Proof by loud assertion has much greater validity.

There seems to be a profound lack of understanding at large for how science works. It may have been the anti-evolutionists who were the first to grasp at this particular straw, but if I see it one more time, be prepared for a less than linear response.
 

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Offline JP

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« Reply #16 on: 11/11/2010 05:04:02 »
Shrunk
What I assert (loudly) that IT'S ONLY A THEORY

;D
 

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Offline Geezer

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« Reply #17 on: 11/11/2010 05:27:52 »
Shrunk
What I assert (loudly) that IT'S ONLY A THEORY

;D

I can assure you that PROOF BY LOUD ASSERTION is a well proven theory.
 

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Offline JP

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« Reply #18 on: 11/11/2010 06:02:50 »
Shrunk
You're louder.  Therefore I rescind my theory about theories being just theories.  QED.
 

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Offline Geezer

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« Reply #19 on: 11/11/2010 06:14:43 »
Shrunk
You're louder.  Therefore I rescind my theory about theories being just theories.  QED.

That's entirely unacceptable. You may only rescind when loudness approaches (asymptotically) a maximum. I will have to seek moderation.
 

Offline sciconoclast

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« Reply #20 on: 11/11/2010 18:59:26 »
You guys are getting a little off topic.

JP; as for your statement that "entanglement is pretty solidly established": what is really well established is that some events produce particles or photons with opposite symmetries.  In classical physics these entities initially appear in these states and there is no need for the principle of entanglement.   

In Bohrs concept of non-locality the symmetries of these entities do not come into realization until a later second interaction.   Bohr introduced the principle of remote entanglement, referred to by Einstein as "fudge", as the only way he could account for the existence of mirror image particles and photons in his theory.

In quantum theory the second interaction that brings the symmetries into realization collapses the quantum field and ends the entanglement.   There is therefore no experimental difference between mirror image particles or photons arising from the collapse of entangled probabilities that are no longer entangled and the classical generation of initial mirror image particles that are un-entangled.

There were a lot of experiments to see if indeed the simultaneous testing for symmetries in paired photons would show pairing.   They did.  Some consider these experiments to disprove quantum theory while others consider that the instantaneous conveyance of information within the quantum field is not an impossibility.

I gather that you are also not making a distinction between Quantum Mechanics and Quantum Theory as I have.

Geezer: science contains a lot of categories including; laws, theories, hypothesis, speculation, topics of inquiry, complied data, etc.   Evolution, which you used as an example has been elevated to law by the American Academy of Science and or the World Academy of Science as well as others; Quantum Theory has not; and String Theory does not yet technically qualify as a theory, although I think it has a lot of promise.   Saying it is only a theory is a more polite way of saying there are other options ( some Nobel Prize Winners have even referred to Quantum Theory as Vodo Physics ).   I followed my statement that it is only a theory with the comment that my objections to Quantum Theory are that it does not meet experimental results.   I am theorizing that you did not actually read my post but where reacting to some catch words in JP's post; but then thats only a theory.

pikaia:  I hope my first post in this tread answered your question as to where the spookyness lies in remote entanglement before all the shouting began.   It must be correct as there hasn't been any correction to my discription.         
« Last Edit: 11/11/2010 22:54:19 by sciconoclast »
 

Offline Geezer

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« Reply #21 on: 11/11/2010 22:51:00 »
What's the difference between Quantum Mechanics and Quantum Theory? I thought the mechanics was based on the theory.
 

Offline sciconoclast

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« Reply #22 on: 11/11/2010 23:26:41 »
Quantum Mechanics consist of proven formulas that can be applied to accurately describe radiation, energy states, subatomic orbits, etc.   Quantum Theory consist of theoretical extrapolations as to the ultimate nature of these phenomena.

Perhaps the term Quantum Theory can be applied to different areas of physics in different context.

It is not the nuts and bolts of quantum mechanic formulas that I question; it is, as farsight says "the weird stuff", such as non-locality and remote entanglement.   And then there is the even weirder stuff such as Neil Bohr's belief that nothing takes place if it does not affect conscious observation.   
 

Offline Geezer

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« Reply #23 on: 12/11/2010 00:32:04 »
Quantum Mechanics consist of proven formulas that can be applied to accurately describe radiation, energy states, subatomic orbits, etc.   Quantum Theory consist of theoretical extrapolations as to the ultimate nature of these phenomena.


Isn't the "theory" simply the, er, theory, on which the formlae are based?

 

Offline JP

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« Reply #24 on: 12/11/2010 04:49:39 »
In quantum theory the second interaction that brings the symmetries into realization collapses the quantum field and ends the entanglement.   There is therefore no experimental difference between mirror image particles or photons arising from the collapse of entangled probabilities that are no longer entangled and the classical generation of initial mirror image particles that are un-entangled.
The point is that there is experimental evidence that the quantum view is correct.  The statistics of the two photons preclude classical models, and this has been measured.  See Bell's inequalities.

Quote
 It must be correct as there hasn't been any correction to my discription.         
The rest of the thread was a correction to your description.  Your description was misinformation presented as fact, so there's not much to debate aside from saying it isn't supported by evidence.  There is a distinction between measurements of quantum vs. classical theory on "entanglement."  The exact interpretation of quantum theory (Copenhagen, Bohmian, many worlds, etc.) is up for debate, but they all predict the same thing from experiments.
 

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